1. Field of the Invention
This invention relates to picture regeneration, and more particularly, to line-by-line picture regeneration from transformed subpicture information.
2. Discussion of the Prior Art
Various techniques have been proposed for reducing the amount of information necessary to define a picture, particularly for use in transmission systems for television or facsimile. For example, it has been found that when information defining elements of a picture is transformed using a Hadamard transform or a Fourier transform into a series of coefficients, a useable picture can be regenerated from selected ones of the coefficients. Since the selected coefficients comprise less data than the original picture information, storage and communications channel bandwidth requirements are reduced. Additionally, when noise in the communications channel affects picture information transmitted as Hadamard or Fourier coefficients, the picture reconstructed from the coefficients is typically more acceptable than a picture reconstructed from noisy untransformed picture information. A more complete description of transforming picture information is given in an article entitled, "Computer Picture Processing" by H. J. Landau and D. Slepian in the May-June 1971 issue of The Bell System Technical Journal.
As is pointed out in the above-mentioned article, a typical method of transforming a picture comprises encoding the picture as a matrix of digitally encoded picture points, dividing the picture into a coarse matrix of subpictures, transforming the amplitudes of the picture points of each subpicture with a Hadamard transform, and selecting certain coefficients resulting from the transformation to represent the picture. The Hadamard transform is preferred for this purpose because it is simpler to implement than Fourier or other transforms.
In applying the Hadamard transform, choices must be made among various decision methods for selecting the coefficients, the weighting factors for balancing the coefficients, and the quantizing methods used to encode the original picture. These choices may influence the quality of the picture reconstructed from the transformed picture information; however, their selection is beyond the scope of this discussion, and not necessary to an understanding of the invention to be described.
Algorithms for Hadamard transformations are well known; for example, a description of such an algorithm appears in the article entitled, "Hadamard Transform Image Coding," by Pratt et al. that appears on page 58 of the January 1969 issue of the Proceedings of the IEEE. The algorithm can be implemented by a general-purpose digital computer or by logic circuitry designed for the purpose as is well known in the art. Logic circuitry would be preferred for a television system because high-speed operation is necessary.
Given that the subpictures comprising a picture have been encoded as selected Hadamard transform coefficients, how is the original picture to be reconstructed? Obviously, one would consider taking the inverse transform of the coefficients to reconstruct the picture points for each subpicture, and then reassembling the subpictures to form a representation of the original picture. However, it is not always convenient to display a picture as a series of subpictures. For example, facsimile receivers and television displays typically reconstruct a picture as a raster of variable intensity lines. The problem exists, then, of converting picture information encoded as Hadamard transform coefficients of subpictures into line-by-line picture information for reconstructing a representation of the original picture.
A solution that might occur to one familiar with the art would be to reconstruct the amplitudes of the picture points for each subpicture from the encoded information, store the resulting amplitudes of the reconstructed picture points in a memory, and then sequentially retrieve the amplitudes for the picture points in each line. However, such a solution would require a memory large enough to store the amplitudes of all the picture points in a row of subpictures, and would require passage of picture information through elements of two logic systems: a first logic system to reconstruct the picture-point amplitudes for each subpicture, and a second logic system to read the stored amplitudes from the memory and rearrange the amplitudes in the proper sequence to generate the line-by-line signal. A solution requiring fewer logic elements and less memory is desired.